1. Field of the Invention
The present invention relates to silica glass for use in optical systems, such as lenses and mirrors, for photolithography using the wavelength ranges of 400 nm or less and of 300 nm or less, and a method of manufacturing such a silica glass.
2. Discussion of the Related Art
In recent years, VLSI chips have been highly integrated and configured to have numerous functions. In the field of logic VLSI, a so-called xe2x80x9csystem on-chipxe2x80x9d scheme, in which a larger system is incorporated on one chip, is becoming more and more popular. Accordingly, finer pattern manufacture and higher integration are required on the silicon wafer or like substrate of such a xe2x80x9csystem on-chipxe2x80x9d scheme. An exposure apparatus, called a stepper, or the like has been used in the photolithography technique for exposing and transcribing a fine pattern of integrated circuits onto a wafer made of silicon or the like.
In the case of DRAMs, as technology develops from LSI to VLSI, the capacity increases from 1Mxe2x86x924Mxe2x86x9216Mxe2x86x9264Mxe2x86x92256Mxe2x86x921G. Accordingly, the minimum line width to be produced by photolithography apparatus should be increased from 1 xcexcmxe2x86x920.8 xcexcmxe2x86x920.5 xcexcmxe2x86x920.35 xcexcmxe2x86x920.25 xcexcmxe2x86x920.18 xcexcm.
To cope with such a trend, a higher resolution and a deeper focal depth are required for the projection lens of the stepper. The resolution and focal depth are determined by the wavelength xcex of exposure light and the numerical aperture (N.A.) of the lenses.
The finer the pattern, the larger the angle of the diffraction light. Therefore, the diffraction light can not be processed unless the lens has a large N.A. Also, the shorter the wavelength xcex of exposing light, the smaller the angle of the diffraction light. Thus, with a shorter wavelength, a relatively smaller N.A. is acceptable.
The resolution and the focal depth are expressed by,
Resolution=k1xc2x7xcex/N.A.
Focal Depth=k2xc2x7xcex/(N.A.)2 (where, k2 and k2 are proportional constants)
According to these formulae, in order to improve the resolution, either N.A. needs to be increased, or xcex needs to be shortened. However, as shown in the above formula, shortening xcex is preferable in terms of the focal depth. Therefore, the wavelength of exposing light has been reduced from the g-line (436 nm) to the i-line (365 nm), and further to excimer laser beams of KrF (248 nm) and ArF (193 nm).
Also, the optical system installed in the stepper is constructed of a plurality of optical members such as lenses. Therefore, even if the transmittance loss at each lens is small, the cumulative effects of all the lenses may lead to decrease in light amount received at the illumination surface. Thus, very high transmittance is required for each of the optical member.
Therefore, for the wavelength band of 400 nm or less, optical glass, which is manufactured by a special method taking into account transmittance loss arising from a combination of optical members, is used. For the wavelength of 300 nm or less, synthesized silica glass or single crystal fluoride, such as CaF2, is used.
As described above, one of the properties of optical members for a photolithography technique that causes deterioration in the image contrast is a transmission loss. The transmission loss is mainly caused by light absorption and light scattering in the optical member.
The light absorption is a phenomenon caused by electron transition due to photon energy incident on the optical member. When the light absorption occurs in the optical member, the absorbed energy is converted to thermal energy. As a result, the volume of the optical member increases and the refractive index and the surface condition change accordingly. In this case, the desired resolution can not be obtained.
With regard to silica glass, in particular, the synthesized silica glass manufactured by the oxy-hydrogen flame hydrolysis method using SiCl4 as a material, there is very small amount of impurity metal. Accordingly, such a glass has superior transmittance with respect to ultraviolet light.
In general, the desired specification for the transmittance of silica glass used for the optical system of precision instruments, such as photolithography-use projection lenses and illumination lenses, is about 0.1%/cm or less in terms of the bulk absorption.
Accordingly, deterioration in the transmittance, which may occur over a short or long period of time (referred to as xe2x80x9csolarizationxe2x80x9d), is required to be within about 0.1%/cm or less.
In the silica glass, especially when it is irradiated by an ArF excimer laser beam, various color centers, such as xe2x80x9cxe2x89xa1Sixc2x7xe2x80x9d (the Exe2x80x2 center) and xe2x80x9cxe2x89xa1Sixe2x80x94Oxc2x7xe2x80x9d (NBOHC), are generated through two-photon processes from defect precursors (xe2x89xa1Sixe2x80x94Sixe2x89xa1, xe2x89xa1Sixe2x80x94Oxe2x80x94Oxe2x80x94Sixe2x89xa1) and SiO2 primary structure (xe2x89xa1Sixe2x80x94Oxe2x80x94Sixe2x89xa1). Such color centers cause deterioration in the transmittance for the wavelength range in use. To deal with such two-photon absorption, increasing of molecular hydrogen concentration in the glass has been proposed in order to improve the durability against laser irradiation of the silica glass.
However, even when such a conventional silica glass, which suppresses two-photon absorption processes, is used for constructing an exposure apparatus, sufficient focusing properties and adequately high enough throughput have not been achieved.
Accordingly, the present invention is directed to a silica glass having superior durability against ultraviolet light and a method for manufacturing the same that substantially obviate the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a silica glass having sufficient focusing properties and throughput without the disadvantages of the conventional art, and a method of manufacturing the same.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the present invention provides a silica glass for use in an optical system for processing an excimer laser beam, the silica glass having a molecular hydrogen concentration of about 5xc3x971018 molecules/cm3 or less and being substantially free from defects which become precursors susceptible to a one-photon absorption process and a two-photon absorption process upon irradiation of the excimer laser beam to the silica glass.
In another aspect, the present invention provides a silica glass having a molecular hydrogen concentration of about 5xc3x971018 molecules/cm3 or less to substantially suppress defects which become color centers through a one-photon absorption process upon irradiation of an excimer laser beam, the molecular hydrogen concentration being greater than an amount that is necessary to substantially suppress defects which become color centers through a two-photon absorption process upon irradiation of the excimer laser beam.
In a further aspect, the present invention provides a method for manufacturing a silica glass for use in optical system for processing an excimer laser beam, the method including the steps of maintaining a silica glass having a molecular hydrogen concentration of more than about 5xc3x971018 molecules/cm3 at a temperature of about 1000xc2x0 C. or more for about 10 hours or more; thereafter cooling the silica glass to a temperature less than 1000xc2x0 C. in a controlled cooling manner; and thereafter leaving the silica glass in an atmospheric condition to cool the silica glass to a room temperature.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.